The so-called "airlay" technique has been known for many years for forming a fiber web. That technique is characterized by individualized fibers being dispersed and projected by means of a flow of air onto an air-permeable surface which enables the fiber web to be formed and transported. The individualized fibers are subjected to random dispersion as they travel in the air flow, thereby contributing to obtaining a fiber web having mechanical properties with improved isotropy.
When implementing the airlay technique, the degree of fiber opening plays a preponderant role in determining the uniformity of the resulting web. It will be understood that the greater the extent to which the fibers are individualized prior to being injected into the air flow, the smaller the risk that the final fiber web will have residues or clumps of fibers stuck together, which give rise to visible localized marks in the web, which spoil the uniformity thereof.
To prepare the fibers prior to injection into the airlay air flow, it has been known for many years to perform a carding operation on the fibers upstream from said air flow. The carding operation is generally implemented by means of a carding system having at least one carding cylinder at its outlet, which cylinder is fitted on its periphery with combing means serving to individualize the fibers and cause them to extend substantially parallel to one another on the periphery of the carding cylinder. By way of non-limiting example, the combing means can be constituted in conventional manner by a working roll associated with a stripping roll, or more usually by a plurality of successive pairs of those two types of roll. The combing means can also be in the form of a fixed housing having a plurality of carding spikes, commonly known as a carding plate. At the outlet from the combing means disposed at the periphery of the carding cylinder, fibers are obtained which are pressed against the surface of the carding cylinder and which are individualized and which extend substantially parallel to one another in the longitudinal direction corresponding to the direction in which the material is advancing.
In a first known method that combines upstream fiber carding with the web-forming airlay technique, an airlay air flow is established to disperse and project the fibers in such a manner that the air flow is oriented substantially tangentially to the last carding cylinder of the carding system and comes into contact with the surface of the cylinder. That first method is already described in numerous publications and corresponds, for example, to the solutions proposed in European patent application EP-A-0 093 585 (CHICOPEE) or in international patent application WO96/06964 (HERGETH HOLLINGSWORTH GMBH). In that first method, the fibers are detached from the carding cylinder both under the effect of the air flow and under the effect of centrifugal force, it being understood that in order to implement the fiber carding functions, the last carding cylinder rotates at high speed. The air flow thus performs two functions: it assists in detaching the fibers from the surface of the carding cylinder; and it directs the fibers, reorienting them in random manner on a remote air-permeable collecting surface for forming and transporting the fiber web. In the particular variant of international patent application WO96/06964, fiber dispersion is further improved by the presence of an additional rotary cylinder (referenced 20 in the figures of application WO96/06964) driven at high speed in the opposite direction to the last carding cylinder (referenced 8 in the figures of application WO96/06964), and placed on the trajectory of the fibers in the air flow.
Proposals have also been made in international patent application WO97/20976 (E.I. DUPONT DE NEMOURS AND COMPANY) for a second method that combines upstream carding of fibers with the airlay web-forming technique. That method is characterized by implementing a disperser rotary cylinder between the last carding cylinder of the carding system and the airlay air flow, which disperser cylinder is driven at high speed and serves to eject the fibers into the airlay air flow under the effect of centrifugal force. In that second method, the fibers are detached from the periphery of the disperser cylinder solely under the effect of centrifugal force without any assistance from the airlay air flow. In a first variant shown in FIG. 1 of that publication, the disperser cylinder (reference 50) takes up the fibers directly from the periphery of the carding cylinder (referenced 40). In another variant, shown in FIG. 3 of that publication, a transfer cylinder referenced (48) is interposed between the disperser cylinder (50) and the carding cylinder (40), with the transfer cylinder commonly being referred to as a "communicator". This cylinder serves merely to take up the fibers from the periphery of the last carding cylinder and to transfer them unchanged to the disperser cylinder, without causing them to be subjected to any kind of transformation, and in particular without altering their parallel orientation.
The first and second above-mentioned methods are advantageous in that they make it possible to obtain fibers with a satisfactory degree of opening by suitably adjusting the action of the carding members, where such adjustment is these days thoroughly mastered. However, in practice, it is found that obtaining a large degree of fiber opening is not sufficient on its own to obtain a uniform fiber web, and that the most important factor affecting the quality of the fiber web lies in the random dispersion of the fibers by the airlay air flow on the surface where the web is formed and transported. The parameters affecting said random dispersion are numerous and at present they are poorly mastered. These parameters include in particular the nature and the length of the fibers, the speed, the width, and the orientation of the airlay air flow, and the speed of rotation of the last cylinder (carding cylinder for publications EP-A-0 093 585 and WO96/06964; disperser cylinder for publication WO97/20976). Adjusting parameters that affect random dispersion of the fibers is thus extremely difficult, and in practice it can only be done experimentally and for any one given type of fiber, by performing successive tests and monitoring the uniformity of the resulting web. Apart from the above-mentioned drawback which is common to the first and second methods, and which is associated with the preponderance of the random dispersion of the fibers under the action of the airlay air flow in obtaining a web that is uniform, the first method also has the drawback of requiring an airlay air flow that is very powerful to enable the fibers to be detached, since in practice they are tightly held to the last carding cylinder. Unfortunately, implementing a powerful air flow gives rise to turbulence which is difficult to master and which spoils the uniformity of the resulting fiber web, in particular by causing fibers to group together preferentially in the form of clumps.
When implementing the second method, the airlay air flow does not serve to detach fibers from the periphery of a cylinder, in this case the disperser cylinder, so the flow can advantageously be weaker than that used in the first above-mentioned method. However, this is at the cost of the need to drive a cylinder (the disperser cylinder) at very high speed, which is expensive in terms of energy consumption, and is more difficult to achieve mechanically speaking, particularly because of vibratory phenomena which may be generated on the cylinder. In addition, the second method recommended in international patent application WO97/20976 also suffers from an additional drawback associated with selecting the peripheral covering of the disperser cylinder. The peripheral covering of the disperser cylinder needs to satisfy two contradictory constraints. Firstly it must be sufficiently aggressive to enable it to take up fibers effectively from the periphery of the upstream cylinder (carding cylinder in the first variant, or "communicator" cylinder in the second variant), thereby avoiding harmful clogging of the upstream cylinder, while also being sufficiently unaggressive to enable the fibers to be ejected under centrifugal force and to avoid fibers being retained on the periphery of the disperser cylinder. It is therefore necessary to select an opening angle and a density per unit area for the teeth of the covering on the disperser cylinder that make it possible to achieve a compromise between those two contradictory constraints. As a result selecting a covering for the disperser cylinder is very difficult and the range is limited.